Variable-pitch multi-blade propeller incorporating individually dismountable blades made of composite materials, process for manufacturing such blades and blades thus produced

ABSTRACT

Each blade is fixed to the hub by the root part of the spar, arranged in the form of a loop surrounding a connection element bolted on the hub, with the interposition of rigid rings by a pin simultaneously fixing the hub on a drive sleeve transmitting thereto the driving torque of the central shaft. The hub comprises two coaxial cylindrical walls in each of which is pierced, for each blade, a circular opening in which the member for controlling the angle of attach which is fast with the blade root is mounted to rotate via self-lubricating rings. The invention is particularly applicable to helicopters.

The present invention relates to multi-blade propellers and moreprecisely to a variable-pitch multi-blade propeller adapted to be usedin particular as tail rotor of a rotorcraft, such as a helicopter, onwhich the rail rotor may be faired.

The invention also relates to a blade and to a process for manufacturingthis blade, which is particularly intended for equipping a multi-bladepropeller.

Variable-pitch multi-blade propellers, which may be used as tail rotorsof the faired type for helicopters, are described in U.S. Pat. No.3,594,097 and in U.S. Pat. No. 4,281,966.

In U.S. Pat. No. 3,594,097, each of the blades of the multi-bladepropeller is connected, by an elongated radial element twistable aboutits longitudinal axis, to a hub driven in rotation by a rotor shaftabout the axis of the propeller, and a fitting of the root of each bladein the hub allows both the rotation of the blade about its longitudinalaxis and a slight angular movement of the blade in flap and in drag,whilst, for controlling the angle of attack, the root of each bladecomprises a crankpin connected, by a ball joint, to a plateperpendicular to the axis of the propeller and axially movable.

Such a propeller, mounted as tail rotor, compensating the drive torqueof a helicopter, in a window or an airflow delimited vertically in therear part of the fuselage or in the vertical stabilizer of thehelicopter by a fairing, is very advantageous insofar as it contributesconsiderable safety during manoeuvres of the helicopter near the ground,and, by reason of the fairing within which the propeller rotates, theblades of the latter are subjected to stresses weaker than those of aconventional tail rotor, which is not faired, this increasing their lifeduration.

However, such a multi-blade propeller presents a certain number ofdrawbacks. In particular, these blades are made of light metal. They aretherefore subject to corrosion, erosion and to fatigue due to alternatestresses. Moreover, machining of the blades is long and delicate andtherefore expensive, and this all the more so as, not only must thesolid part of the blade subjected to the aerodynamic stresses be madewith a profile and in accordance with a precise law of twist, exactlyreproducible from one blade to the other, while being protected at theleading edge by employing a process of superficial hardening such ashard anodization, but also the blade root is likewise delicate andexpensive to produce.

In fact, the blade root comprises, on the one hand, a hollowintermediate part in which are machined an axial recess adapted toreceive the outer radial end equipped with a bushing of the elongatedelement connecting the blade to the hub, and a transverse bore openingin the axial recess and adapted to receive a shaft passing in thebushing and ensuring fastening of the blade root on the elongatedelement, and, on the other hand, a cylindrical end bearing, whose outersurface is superficially hardened by chromium-plating or anodization andwhose central part is thinned, and which is adapted to allow rotation ofthe blade, about its longitudinal axis, to control the pitch.

To this end, this cylindrical bearing is mounted to slide and rotate ina radial sleeve made of relatively supple synthetic material with a lowcoefficient of friction, which is embedded over about half of its lengthtowards the axis of the propeller in a radial cylindrical bore machinedin the hub, and free towards the outside, each sleeve being held inplace by a collar which is housed in a corresponding groove in the boreand immobilized in rotation by a stud passing through the wall of thebore. In order to effect such an assembly of the blade root, whichconstitutes a sort of semi-embedding on the hub, it is indispensablethat the latter presents a rim having a large radial thickness, with theresult that sufficiently long cylindrical bores may be pierced in thisrim to serve as housing for the sleeves, which is disadvantageous fromthe standpoint of weight, manufacturing costs and the centrifugalefforts which stress this important eccentric mass constituted by twocircular cheeks bolted one on the other.

Moreover, the disc or bottom of the hub, which connects the rim of thehub to the shaft of the rotor by being connected to a radial flange ofthe latter by a ring of bolts, must be of a thickness corresponding tothe thickness of the rim, and this all the more so as, by reason of theassembly of the blade roots on the hub, the latter is subjected tobending stresses as well, moreover, as the bolts for fixing the hub tothe shaft, as the bolts serve simultaneously for fixation on the shaftof the inner radial part of the elongated elements for connecting theblades to the hub. The hub therefore does not serve solely to transmitthe drive torque from the rotor shaft to the blades, but it is activelystressed by forces which are transmitted thereto by the blades and theelongated connecting elements, up to its part connected to the rotorshaft, and it must therefore be dimensioned accordingly.

Furthermore, the crankpin for connecting the root of each blade to theaxially movable plate in order to control the angle of attack isconstituted by a lateral lever, fast with the inner radial end of thecylindrical bearing, and bearing a shaft provided with a spherical ringmade of elastomer which just fits inside the eye of an axial yoke fastwith the radial periphery of the plate; but nothing positively retainsthe elastic ring in the eye of the yoke. Finally, the elongatedtwistable element connecting each blade to the central part of the hubsurrounding the shaft of the rotor is constituted by one of the arms ofa star-shaped member comprising as many arms as the propeller comprisesblades, all the arms being fast with one another by a central flat ringof this member, by which this member surrounds the shaft of the rotorand is connected to the latter by the ring of bolts. This star-shapedmember is made by superposing a plurality of discs of thin sheet steelwhich are cut out in star form, with the result that each arm is formedby a bundle of thin leaves joined together, the outside leaves of thebundle being of smaller width than those of the inner leaves in order todistribute the torsional stresses uniformly between the different leaveswhich are, moreover, coated with an anti-friction plastic coating inorder to avoid corrosions caused by contact.

This star-shaped metal member, which must be produced carefully andwhich is therefore expensive, is subjected to much stress and it must beregularly changed after a certain time in service or if it has beendamaged.

Such a multi-blade propeller is therefore a member which is relativelyexpensive to manufacture and to maintain.

In U.S. Pat. No. 4,281,966, there is described a variable-pitchmulti-blade propeller of simplified type, which overcomes a certainnumber of drawbacks associated with the embodiment set forth above andwhich essentially concern the limited service life of essentialcomponents, such as the star-shaped member and the blades, as well asthe general complex architecture of the multi-blade propeller.

In the multi-blade propeller forming the subject matter of U.S. Pat. No.4,281,966, two diametrically opposite blades are essentially constitutedon the one hand by a common spar, made by a single elongated leaf offibers with high mechanical resistance agglomerated by a syntheticresin, the leaf being fastened by its centre to a hub and, on the otherhand, by two shells with aerodynamic profile fixed to the leaf on eitherside of the hub, the inner end of each shell being fast with a memberfor controlling the angle of attack of the corresponding blade, which isarranged so as to exert on the shell a torsional moment centred on thelongitudinal axis of the elongated leaf. The latter is formed by twoflat bundles of fibers, which are disposed so as to form at the centreof the leaf an opening allowing free passage of a shaft for collectivelycontrolling the angle of attack of all the blades, this shaft being, asis the case in the embodiment described in U.S. Pat. No. 3,594,097,mounted to slide axially in the shaft of the rotor which is tubular.Moreover, the shell of each blade is joined over the whole of its lengthto the corresponding part of the leaf by a mass of adhesive syntheticmaterial, cellular or foam, of low density and possibly presenting acertain residual elasticity. Each shell is essentially formed by layersof glass fiber fabrics and by a leading edge in stainless steel sheet,the whole being agglomerated by a synthetic resin polymerized so as tointegrate the leading edge in the layers of fabrics.

The member for controlling the angle of attack is constituted by asleeve cast in glass fiber reinforced synthetic material and which isconnected to the root of each blade. A bearing surface, constituted by acircular metal ring, is embedded in or glued on the inner cylindricalend of the sleeve, coaxially ot the longitudinal axis of the blade, anda metal pitch control finger is embedded in a lateral arm borne by theinner end of the sleeve. By its bearing surface, each blade is mountedto rotate in a bearing of cast self-lubricating material, resting insemi-circular recesses made on the periphery on the one hand of anannular casing in the form of a dish, fast with the hub of the propellerand with its drive shaft, and on the other hand of a cover whichsurmounts the casing. The end of the shaft for collectively controllingthe angle of attack of the blades, which passes through the hub, bears aplate substantially perpendicular to this axis and to which are fixedpairs of lugs provided with holes adapted for the engagement of thecontrol fingers of the sleeves of two adjacent blades.

The advantages of this embodiment are that the structure of the devicecontrolling the angle of attack is very simple and that each blade isalso of simple, robust and light structure, virtually insensitive to thephenomena of erosion and to the notch effect, with the result that theirservice life is virtually unlimited. The multi-blade propeller thusproduced is of much lower cost price and is much lighter than thepropellers with metal blades. Moreover, its maintenance costs are muchlower.

However, in the course of landing or take-off on unprepared areas,stones may penetrate in the airflow in which the propeller rotates andthey may damage the blades thereof, further to which it is necessary todismantle these blades in order to proceed with repair and/orreplacement thereof. However, as the blades are arranged in pairs ofopposite blades, having, for each pair, a common elongated leaf, atleast one pair of blades for each deteriorated blade must be dismantled.This operation is extremely complex as the hub of the propeller isformed by a bonded stack comprising, in superposition, the median partsof all the elongated leaves as well as two outer layers of glass fiberfabrics impregnated with a polymerized synthetic resin, the whole of thestack being embedded in a synthetic filling material, and the hub thusconstituted being fixed to the tubular drive shaft of the rotor by pairsof bolts which pass through the periphery of the bonded stack, on eitherside of the median part of each elongated leaf.

Dismantling of a pair of blades is therefore a major drawback, as it canonly be carried out by taking apart the stack which constitutes the hub.This can only be carried out in the workshop, after having removed thecover and disconnected from the shaft of the rotor the assembly of theblades and the hub as well as the housing, and the blades from the lugsof the pitch control plate, and after having disconnected this platefrom the shaft for collectively controlling the pitch in order todismantle the propeller proper.

In order not to immobilize the helicopter on the ground for a prolongedperiod, it is necessary to reassemble a spare propeller, which supposesthat the operator has such a spare propeller available. The necessity ofreplacing the blade and hub assembly as soon as one of the blades isdamaged also involves, in this embodiment, considerable financialinvestment.

Finally, this embodiment must necessarily comprise an even number ofblades, since these latter are fast in two's by common spars, which isdisadvantageous, since the multi-blade propellers used as tail rotor offaired type have a much lower sound level when they comprise an oddnumber of blades.

It is an object of the present invention to propose, whilst conservingthe advantages presented by the second embodiment described hereinaboveover the first, to overcome the drawbacks of this second embodiment,thanks to a multi-blade propeller whose general architecture allowseasy, rapid assembly, dismantling and maintenance of the differentcomponents of the propeller, and particularly of its blades, of whichthere is preferably an odd number, and which, when they present astructure according to the invention, improve reliability and reducevulnerability of the multi-blade propeller which they equip.

To this end, the variable-pitch multi-blade propeller according to theinvention, intended in particular to be used as tail rotor ofrotorcraft, and which comprises:

a central shaft driven in rotation about a central axis,

a hub rotating with the central shaft about said axis,

blades, preferably in an odd number, each comprising

a shell with aerodynamic profile constituted by at least one layer offiber fabrics with high mechanical resistance rigidified by apolymerized synthetic resin for impregnation, and of which the inner endextends by a blade root.

a filling body made of cellular or foam synthetic material disposed inthe shell,

a spar whose longitudinal axis is parallel to that of the blade andconstituted by a single elongated leaf of fibers with high mechanicalresistance agglomerated by a polymerized synthetic resin, of which themajor part is fixed in the shell and of which one end part, emergingfrom the shell on passing through the blade root, forms a twistable andflexible root part by which the spar is connected to the hub,

and such that the blade root is fast with a blade pitch control member,which is adapted to exert on the shell a torsional moment, substantiallycentred on the longitudinal axis of the spar, when this member isactuated by an assembly for collectively controlling the angle of attackof the blades, this member being, moreover, mounted to rotate in the hubabout the longitudinal axis of the corresponding blade, wherein eachblade is individually connected to the hub by the root part of the spar,which is arranged in a loop surrounding, by its inner end, a singleconnection element bolted on the hub.

Such a propeller allows easier maintenance insofar as each damaged blademay be dismantled rapidly and changed without it being necessary todismantle other components of the propeller. In fact, each connectionelement if preferably axially traversed by the shank, with threaded end,of a single pin for connecting the connection element to an annular,flat, radial part of the hub, surrounding a central opening made in thelatter to allow the coaxial arrangement of the hub about the centralshaft.

Two rigid, radial, flat rings are advantageously disposed on either sideof all the connection elements of the different blades and about thecentral shaft, and each present, for each connection element, a boreadapted to be aligned with the central passage of the correspondingconnection element and to receive the shank of the single pin forconnecting the connection element to the hub, in order to distribute thecentrifugal forces from one connection element to the other. Bytransmitting the efforts from one of the connection elements to theother, these rings make it possible not to bend the shank of the singlepin for connecting the connection element of a blade to the hub.

In order to ensure good support of the blade on the hub, the latter isadvantageously in the form of a dish comprising two coaxial cylindricalwalls radially spaced apart from each other and in each of which ispierced, for each blade of the propeller, a circular opening coaxial tothe corresponding opening of the other wall and each centred on thelongitudinal axis of the corresponding blade, the blade pitch controlmember being mounted to rotate in each of these two openings via aself-lubricating ring coaxial to the opening in which the ring ishoused.

The blade pitch control member may be assembled under good conditions ifthe internal diameters of the opening made in the inner radial wall ofthe hub and of the self-lubricating ring housed in this opening arerespectively less than the inner diameters of the opening pierced in theouter radial wall of the hub and of the self-lubricating ring housed insaid latter opening, the member controlling the angle of attack of thecorresponding blade presenting two coaxial circular bearing surfacescentred on the longitudinal axis of the blade, radially spaced apartfrom one another so that by its outer radial bearing surface, of whichthe outer diameter corresponds to the inner diameter of the outer radialring, and by its inner radial bearing surface, of which the outerdiameter corresponds to the inner diameter of the inner radial ring,this member may abut respectivley inside the outer and inner radialrings.

To ensure good connection with the root of the blade, this member forcontrolling the angle of attack is preferably a bearing in the form of asleeve, traversed by the root part of the spar of the blade, coveringthe blade root and connected to the latter, for example by gluing, andof which each of the ends is surrounded by one of the two bearingsurfaces.

If the blade root comprises a hollow cylinder whose inner end istruncated in form and a transition zone connecting the hollow cylinderto the shell of the blade, the sleeve comprises, in an embodimentparticularly well adapted to its cooperation with the blade root, anouter cylindrical part of large diameter surrounding the hollow cylinderof the blade root and connected by an intermediate truncated part, bywhich the sleeve bears against the truncated inner end of the hollowcylinder, to an inner cylindrical part of small diameter.

Where the sleeve is made of metal and in one piece, it is advantageousif the two bearing surfaces are each constituted by one of two collarsof spheroidal shape on the outer surface of the ends of the sleeve.

In order to reduce the radial efforts on the self-lubricating rings, itis advantageous if the member controlling the angle of attack alsocomprises, between its two bearing surfaces spaced apart from each otherto a maximum, a lateral boss connected by a ball joint to one of theaxial lugs, in a number equal to the number of blades, of a plate forcontrolling the angle of attack displaced axially, parallel to the axisof rotation, by a shaft for collectively controlling the angle ofattack, mounted to slide axially in the central shaft which is tubular.

The propeller advantageously comprises, in addition, a driving sleeve,adapted to transmit the driving torque from the central shaft to thehub, and surrounding the central shaft, while being coaxial to thelatter and rotating with the latter, this driving sleeve comprising anannular, outer radial flange for connection to an annular radial part ofthe hub.

In this case, the connection of the flange of the driving sleeve to theannular part of the hub may advantageously be ensured by the pins which,regularly distributed about the axis of rotation and in a number equalto the number of blades of the propeller, each simultaneously ensureretention, on this annular part of the hub, of a connection element ofthe root part of the spar of a blade.

If the drive of the central shaft is ensured by outer grooves thereon,by which it is in mesh on the one hand with inner grooves on a drivinggear and on the other hand with inner grooves on the driving sleeve,which is separated from the gear by a seal-holder, it is advantageous,in order to avoid axial clearances, vibrations and deterioration of theelements of the axial stack constituted by the driving sleeve, theseal-holder and the driving gear, if an axial prestress device makes itpossible to maintain a predetermined axial bearing force of the drivingsleeve against the seal-holder and against the driving gear. Where thedriving sleeve is maintained in axial position about the central shaftwith the air of a tapped ring screwed at the end of the central shaft,the prestress device may comprise screws, screwed in tappings passingthrough the ring parallel to its axis, and of which the end of the shankof each is shaped as a bearing stud applied against a bearing surface atthe bottom of a notch in the axial end face of the driving sleeve whichis opposite the seal-holder.

It is a further object of the present invention to provide a bladeadapted to equip a multi-blade propeller, and in particular a propellerwhich may be used as tail rotor of a rotorcraft.

Such a blade is described not only in the U.S. Pat. No. 4,281,966mentioned above, but also in U.S. Pat. Nos. 3,647,317 and 4,306,837.

The first of these last mentioned two Patents describes an axial-flowventilator propeller for cooling towers and heat exchangers in general,and this propeller is equipped with glass fiber blades. Each blade isessentially constituted by a profiled shell of glass fiber fabricsfilled with a high-density polyurethane foam and in which is disposed asteel spar presenting a H-section and embedded in the filling foam towhich the spar is chemically bonded.

Such a blade is advantageous in that its glass fiber skin offersexceptional resistance to corrosion and abrasion under severeenvironmental conditions. Moreover, the high-density polyurethane foamreinforces the profile over the whole span and efficiently transfers theloads applied on the skin to the spar, increasing the torsionalstability and resistance to impacts of the blade.

However, this blade presents a major drawback, connected with the natureand shape of its spar, which is expensive to manufacture and iscumbersome, with the result that this solution cannot be used for makingblades of small thickness and light weight.

In the second of the last mentioned two U.S. Patents mentioned above,which relates to a helicopter tail rotor with two diametrically oppositeblades connected to each other by a common spar, each blade comprises askin surrounding a preformed honeycomb filling body, the spar beingconstituted by a thin, flat strip of unidirectional fibers of highmechanical resistance, for example graphite, KEVLAR or glass, common tothe two opposite blades and extending over the whole of their span, andwhich presents a relatively thick median part, forming the hub, to whichare adjacent two finer, twistable and flexible parts which each extendby two end parts themselves each divided into two half-spars of whichone is an upper surface half-spar and the other a lower surfacehalf-spar each extending just beneath the corresponding skin part of theblade, between this skin and the filling body.

Likewise in this embodiment, the major drawback lies in the structure ofthe spar, common to the two opposite blades and in the form of anelongated, recumbent H, which is therefore complex and expensive toproduce,

The present invention proposes to produce a blade which presents thesame advantageous features as those described in the state of the art,but whose structure is simpler and which is easier to produce.

To this end, the blade according to the invention, comprising:

a shell with aerodynamic profile, constituted by at least one layer offiber fabrics with high mechanical resistance rigidified by apolymerized synthetic resin for impregnation,

a filling body made of a cellular or foam synthetic material, disposedin the shell,

a spar whose longitudinal axis is substantially parallel to that of theblade and constituted by a single elongated leaf of rovings of fiberswith high mechanical resistance agglomerated by a polymerized syntheticresin, such that the major part of the leaf is fixed in the shell and ofwhich an end part of the leaf, emerging from the shell, forms atwistable and flexible root part by which the spar is adapted to beconnected to a hub, and

preferably, a metal leading edge cover integrated in the shell, is suchthat the preformed filling body comprises a cut-out which extends overthe whole length of this body, which opens in the face of the bodyturned towards the upper surface part of the shell and whose sectioncorresponds substantially to that of the spar in that part thereof whichis fixed in the shell, and so that the part of the spar which is fixedin the shell is disposed in the housing defined by the cut-out in thefilling body and the upper surface part opposite the shell, and isdirectly fixed by its face turned towards the upper surface against thisupper surface part of the shell.

The particularity of this assembled structure constituted by the shell,the filling body and the spar resides in the direct bond of one face ofthe spar, over the whole length thereof which is included in the shell,with the coating fabrics of the upper surface, which procures a bond ofbetter quality between the spar and the coating by a direct bonding ofone element on the other by a large surface, which leads to a betterresistance to the centrifugal force, to the bending moments and to thetorsional moment.

In order to ensure good torsional rigidity of the whole of the blade,the shell is constituted by a stack, from the outside to the inside, ofat least one layer of fabrics of glass fibers or KEVLAR but preferablyof two layers which are crossed and inclined by 45° with respect to thelongitudinal axis of the blade, and of at least one layer of carbonfiber fabrics, but preferably two layers likewise crossed and inclinedin similar manner.

The join, at the trailing edge, of the upper surface and lower surfacefabrics forming the shell is advantageously reinforced by an inner yokemade of carbon fiber fabrics, bonded by its outer faces on the innerfaces of the fabrics of the shell.

In order to simplify the connection of the blade to the hub, the rootpart of the spar arranged as a loop advantageously surrounds aconnection element of the glade by its end opposite that spar part fixedin the shell.

In a particularly simple embodiment, the elongated leaf forming the sparis constituted by a single bundle of rovings which is folded on itselfin two equal halves of which the inner parts adjacent the zone of fold,constitute the root part of the spar and of which the outer parts,remote from the zone of fold, are coupled to each other and constitutethat part of the spar received in the shell.

The root of the blade, which is traversed by the root part of the spar,is preferably made with extensions of the or each layer of fiber fabricsconstituting the shell and covering a stack of layers of fabrics ofreinforcing fibers. This blade root is for example constituted by ahollow cylinder connected by its outer end to the general part of theblade by a transition zone and of which the inner end presents atruncated form, this facilitating the connection of the blade by itsroot on a member for controlling the angle of attack, covering the bladeroot.

It is also an object of the invention to provide a process formanufacturing such a blade, by means of a lower half-mould and an upperhalf-mould of which the complementary impressions have the form of thelower surface part and of the upper surface part of the blade,respectively.

The process according to the ivention consisting in depositing in thelower half-mould a foil of at least one layer of fabrics of fibers ofhigh mechanical resistance, impregnated with a polymerizable syntheticresin and adapted to form the lower surface part of the shell of theblade, is such that it further consists in positioning, above this orthese layers of fabrics, a preformed filling body made of a cellular orfoam synthetic material presenting over the whole of its length acut-out which opens in the face of the body turned towards the uppersurface, in covering the rear edge of this filling body with a preformedyoke made of fabrics of carbon fibers, in disposing in the cut-out apart adapted to be fixed in the shell of a single elongated leaf ofrovings of fibers of high mechanical resistance agglomerated by apolymerizable synthetic resin and adapted to constitute the spar of theshell, and of which an end part, adapted to form a root part of thespar, is disposed beyond a corresponding end of the filling body, indisposing on the assembly thus formed in the lower half-mould a foil ofat least one layer of fabrics of fibers with high mechanical resistanceimpregnated with polymerizable synthetic resin and adapted to form theupper surface part of the shell of the blade, in preferably installing ametal cover at the leading edge in the lower half-mould, and finally inplacing the upper half-mould on the lower half-mould and in polymerizingthe or each resin impregnating and/or agglomerating the elementsenclosed in the mould.

In the preferred variant embodiment, which makes it possible to mouldand polymerize in a single mould and in one operation the general partof the blade, the part of spar fixed in this general blade part, and theroot part of the spar, the process according to the invention consistsin using a single thermosetting synthetic resin for impregnating thefoils of each of the layers of fiber fabrics of the shell and of thetrailing edge yoke and for agglomerating the rovings of the singleelongated leaf of the spar, respectively after having cut out each layerin the form of an elongated layer comprising two adjacent foils oneither side of a longitudinal median axis corresponding to the leadingedge of the shell and adapted respectively to form the lower surfacepart and the upper surface part of the shell, and after having spread asingle bundle of rovings folded on itself in two equal halves, in orderto form the single leaf of the spar, and the process according to theinvention further consists in leaving the foil of the upper surface partof each layer outside the lower half-mould when the foil of the lowersurface part of the corresponding layer is deposited in the impressionof the lower half-mould, then, after having positioned the filling body,the trailing edge yoke and the bundle of rovings, in folding down thefoil of the upper surface part of each layer onto said body and on saidbundle, before installation of the leading edge cover, the closure ofthe mould and a heat treatment for polymerizing the resin.

The invention will be more readily understood on reading the followingdescription with reference to the accompanying drawings, in which:

FIG. 1 is a horizontal section through the rear part of the fuselage ofa helicopter equipped with a anti-torque rotor according to theinvention.

FIG. 2 is an axial half-section, along II--II of FIG. 3, of the rotorshown in FIG. 1.

FIG. 3 is a radial quarter section along III--III of FIG. 2.

FIG. 4 is a transverse section through a blade along IV--IV of FIG. 3.

FIG. 4a is a view on a larger scale of the trailing edge of the blade ofFIG. 4, and

FIGS. 5 to 7 illustrate the process for manufacturing each of the bladesaccording to FIG. 4, equipping the rotor according to FIGS. 1 to 3.

Referring now to the drawings, FIG. 1 shows the multi-blade rotor usedas anti-torque tail rotor, compensating the driving torque transmittedto at least one main rotor by a main gear box, and generally designatedby 1, which is mounted and driven in rotation in a vertical airflow orwindow 2, of slightly truncated form, defined in the rear part of thefuselage or stabilizer 3 of a helicopter by a fairing 4. In the airflow2, the rotor 1 is supported and driven by an auxiliary gear box 5 housedin a double-walled casing, generally cylindrical in form, maintained atthe centre of the airflow 2 by a support comprising a plurality offaired radial arms 6.

The auxiliary gear box 5, which is for example such as the one describedand shown in U.S. Pat. No. 3,594,097, to which reference mayadvantageously be made for further details, contains a bevel gear whoseinput gear is driven by a transmission shaft 7 connecting the main gearbox to the auxiliary gear box 5, and passing through a hollow arm 8connecting the fairing 4 to the casing of the auxiliary gear box 5. Thelatter also contains a bevel gear 9, stressed by a connecting rod 10 forcollectively controlling the angle of attack in order to displace ashaft for collectively controlling the angle of attack, describedhereinafter, parallel to the axis of the airflow 2. The bevel gear 9 andthe connecting rod 10 have been schematically shown as in FIG. 1 of thePatent mentioned above, for the purposes of clarity, but the connectingrod 10 may possibly also pass through the hollow arm 8.

With reference to FIGS. 2 and 3, the rotor 1 comprises a tubular centralshaft 11 coaxial to the airflow 2, and mounted to rotate about the axisA of the airflow 2 by roller bearings housed in the auxiliary gear box5. The shaft 11 is driven in rotation by axial grooves 12 in a part ofshaft passing through the casing 13 of the auxiliary gear box 5, andwhich are in mesh with corresponding grooves on the secondary outputgear 14 of the bevel gear housed in the auxiliary gear box 5, thissecondary gear 14 itself being mounted to rotate in the casing 13 by aroller bearing 15. By its grooves 12, the shaft 11 is also in mesh withcorresponding grooves on the inner axial end (i.e. facing the auxiliarygear box 5) of a driving sleeve 17 which is in axial abutment by thisinner axial end against a seal-holder 16, clamped against the secondarygear 14. This sleeve 17, rotating with the shaft 11, is coaxial to thelatter thanks to two inner cylindrical bearing surfaces 18 and 19 offsetaxially and such that the threaded outer axial end 20 of the shaft 11projects with respect to the outer axial cylindrical bearing surface 19of the sleeve 17. At its inner axial bearing surface 18, this sleeve 17presents an outer radial annular flange 21 by which the sleeve 17 isfixed to the hub described hereinafter and to which the sleeve 17transmits the driving torque received from the shaft 11, which receivesit from the bevel gear 14, and the sleeve 17 also takes up part of thethrust efforts developed by the rotation of the blades describedhereinafter. The sleeve 17 is maintained axially in place, by exerting apredetermined axial bearing force against the seal-holder 16, by atapped ring 22, screwed on the threaded end 20 of the shaft 11, andwhich is traversed by four axial bores regularly distributed over thering 22 and in each of which is screwed a setting screw 23 of which theend of the shank, in the form of a bearing lug 24, is applied against abearing surface at the bottom of a notch 25 made in the outer axial endface of the sleeve 17.

By applying a predetermined tightening torque on the heads of the fourscrews 23 which abut on the one hand on the ring 22 and on the otherhand on the sleeve 17, an axial prestress is easily exerted on the axialstack constituted by the sleeve 17, the seal-holder 16 and the bevelgear 14, which makes it possible to take up the axial clearances and toavoid the vibrations and deterioration of the elements of this stack,the four screws 23 then being braked simultaneously.

The hub is substantially in the form of a circular cake mould providedwith a central shaft. More precisely, the hub comprises a hub body 26,made of stamped sheet metal or an injected and possibly reinforcedsynthetic material, having the form of a dish whose bottom isconstituted by a flat outer radial part 27 connected by a truncatedintermediate part 28, directed towards the inside of the dish, to aflat, annular, inner, radial part 29 surrounding a central opening ofdiameter slightly larger than the outer diameter of the part of thesleeve 17 located axially outside the flange 21, so that the hub body 26may be slid axially on this part of the sleeve 17, until it rests by theflat annular part 29 against the flange 21 of the sleeve 17. The hubbody 26 is also constituted by two cylindrical walls coaxial to eachother, of which one, 30, in outer radial position and connected by itsinner axial end to the outer radial end of part 27 of the bottom, has aheight greater than that of the other cylindrical wall 31, in innerradial position and connected by its inner axial end to the truncatedpart 28 of the bottom. In these two walls 30 and 31 are pierced, foreach of the blades which the rotor must comprise, two coaxial circularopenings 32 and 33 centred on a radial axis and such that the opening 32in the outer wall 30 has a diameter larger than the diameter of theopening 33 in the inner wall 31. Two self-lubricating rings 34 and 35are housed respectively in annular grooves made in the wall of theopenings 32 and 33, the inner diameter of ring 34 being greater thanthat of the ring 35.

The rotor also comprises eleven blades 36 each being substantially 40 cmin length, 7.7 cm in width and having a law of twist of about 7°, andthe structure of these blades 36 is described hereinbelow with referencenot only to FIGS. 2 and 3 but also to FIGS. 4 and 4a.

Each blade 36 is essentially constituted, in its general part, by ashell 37 having the desired aerodynamic profile, by a preformed fillingmass 38, a trailing edge yoke 72, a spar 39 and a leading edge cover 40made of titanium or stainless steel.

The shell 37 is made of a stack constituted, from the ouside to theinside, by two superposed layers of fabrics of glass or KEVLAR(registered trademark) fibers disposed such that, for example, theirwarp yarns are crossed at right angles and inclined by 45° with respectto the longitudinal axis of the blade, and by two layers of carbon fiberfabrics, preferably likewise crossed and inclined at 45°, and theassembly of these four layers is agglomerated by a synthetic resinpolymerized by heat-setting, so as to form a hollow body ensuringtorsinal rigidity of the whole of the blade, in which the filling mass38 does not present any resistance to shear.

This filling mass 38 is an element of foam or cellular syntheticmaterial, of which the outer shape corresponds to the inner volume ofthe shell 37 in which it is enclosed, and which present a cut-out 41 ofsubstantially rectangular section extending over the whole of its lengthand opening in the face of the mass 38 which is turned towards the uppersurface of the blade 36.

A reinforcing yoke 72, preformed as a V, constituted by carbon fiberfabrics preimpregnated with synthetic resin, covers the rear edge 38a,on the trailing edge side of the blade 36, of the filling mass 38. Thisyoke 72 is bonded by its outer faces to the fabrics constituting theshell 37 and by its inner faces to the filling mass 38. This yoke 72 isessentially intended to ensure resistance of the shell 37 to thetorsional moment and to the drag moments which stress it.

The spar 39 is a solid elongated leaf made of KEVLAR rovingsagglomerated by a heat-setting synthetic resin and having, over abouttwo thirds of its length which are received in the general part of theblade 36, a section which corresponds to that of the housing defined bythe cut-out 41 of the filling mass 38 and by the lower face of theopposite part of the upper surface of the shell 37.

The spar 39 adapted to take up the centrifugal forces stressing theblade 36 in service constitutes, by the last third of its length whichemerges from the general part of the blade, passing through the bladeroot described hereinbelow, a root part 42 in the form of a loop (cf.FIG. 3) whose thickness increases (cf. FIG. 2) from that part of thespar 39 received in the general part of the blade 36 towards the freeend of the loop, which surrounds a metal element or spool 43 formingconnection for the blade 36, being wound in a groove around thisconnection element 43.

In the general part of the blade 36, the particularity of the assemblyof the shell 37, the filling mass 38 and the spar 39 lies in the directbond of the face of the spar 39 which is not opposite one of the facesof the cut-out 41 in the filling mass 38 with the inner face of theinner layer of the coating fabrics of the upper surface part of theshell 37, over the whole length of the spar 39 which is received in thegeneral part of the blade 36, i.e. over about the outside two thirds ofthe length of the spar 39.

This particular structure of the blade 36 is doubly interesting as itprocures a connection of excellent quality between the spar 39 and theshell 37 by a direct bond of one element on the other over a largesurface, which creates a better resistance to the centrifugal force, tothe bending moments and to the torsional moment which stress the blade,and as the manufacture of the blade 36 in accordance with a processwhich will be described hereinafter is facilitated.

Each blade 36 further comprises a blade root 44, made with the layers offiber fabrics of the coating or of the shell 37 and with layers ofreinforcing fiber fabrics 45, in order to give the blade root 44 asufficient thickness. This blade root 44 comprises a hollow cylinder 46,of which the inner radial end narrows into a frustrum of cone, and whichis connected by its outer radial end to the general part of the blade 36by a transition zone 47 extending over a length close to the chord ofthe blade 36. The blade root 44 is mounted by its hollow cylinder 46 ina bearing constituted by a metal sleeve 48 of aluminium alloy.

This sleeve 48 comprises two coaxial tubular parts 49 and 50, ofcircular section having different inner and outer diameters, and ofwhich the cylindrical bores are connected by a truncated part. Thehollow cylinder 46 of the blade root 44 and its truncated inner radialend are respectively covered by the tubular part 49 of larger inner andouter diameter and by the truncated part of the sleeve 48, and arerespectively fixed to these two parts by adhesion. This form of thesleeve 48 promotes support thereof against the hollow cylinder 46 of theblade root 44 under the effect of the centrifugal force.

Around its inner radial end, facing the shaft 11 of the rotor, the part50 of smaller inner and outer diameter of the sleeve 48 presents acollar 52 of spheroidal form whose maximum outer diameter corresponds tothe inner diameter of the self-lubricating ring 35 housed in the opening33 of the inner wall 31 of the hub 26. Similarly, a collar 51, likewiseof speroidal form, and whose maximum outer diameter corresponds to theinner diameter of the self-lubricating ring 34 housed in the opening 32of the outer wall 30 of the hub 26, is presented by the part 49 oflarger inner and outer diameter of the sleeve 48, about its outer radialend, i.e. in the position of greatest distance vis-a-vis the collar 52on the sleeve 48.

The latter also comprises a lateral boss 53, connected to the truncatedpart of the sleeve 48, between the two collars 51 and 52, and extendingin a direction substantially perpendicular to the axis of the sleeve 48as well as to the axis A of rotation of the rotor.

The dimensions and bulk of the sleeve 48 and of its lateral boss 53 aresuch that, by suitably inclining a blade 36 and its sleeve 48 afterhaving introduced the inner radial end of the loop of the root part 42and the metal connection element 43 as well as part 50 of the sleeve 48in an opening 32 in the outer wall 30 of the hub, it is possible to passthe lateral boss 53 in this opening 32, then to engage part 50 of thesleeve 48 in the corresponding opening 33 and to position parts 49 and50 so that the collars 51 and 52 come respectively in abutment insidethe rings 34 and 35 of the openings 32 and 33.

In this position of the blade 36, the boss 53 is opposite one of aplurality of axial lugs 54, in a number equal to the number of blades 36of the rotor, and of which the outer axial end is fast with an annular,convex control plate 55. By a ring of screw-nut assemblies 56, the innerradial edge of the control plate 55 is superposed and connected to theouter radial edge of a cheek 57 of truncated form, in order to presentexcellent rigidity, and which is mounted by conical fit of its centralpart on a conical spindle 58 of a shaft 59 for collectively controllingpitch, mounted to slide axially in the tubular shaft of the rotor 11,the cheek 57 being retained on the pitch control shaft 59 by a flangednut 60 screwed on the threaded outer axial end of the spindle 58.

Each of the axial lugs 54 of the plate 55 is pierced with a boreequipped with a ring 61 in which is received a ball joint 62 retained bya transverse pin on the shank and against the head of a pin 63, of whichthe threaded end of the shank passes through a bore in the boss 53 andreceives a nut 64 cottered on the shank in screwed position.

Each boss 53 constitutes with the pin 63, the nut 64, the ball joint 62and the lug 54 which correspond thereto, a lever for controlling thepitch of the blade 36 of which the root 44 is fast with the corespondingsleeve 48. The different pitch control levers thus contituted, thecontrol plate 55, the cheek 57 and the shaft 59 for collectivelycontrolling pitch, constitute an assembly for collectively controllingthe pitch of the blades 36 of the rotor, such that any axialdisplacement of the shaft 59 in the tubular shaft 11 controls, via thepitch control levers, the application of a torsional moment on thesleeves 48, and therefore also on the blade roots 44 and the blades 36about the longitudinal axes of these latter. This torsional momentprovokes rotation of the sleeves 48 and therefore also of the blades 36about the longitudinal axes of these latter, causing the root part 42 ofthe spar 39 to twist. Each pitch control lever is located between thetwo collars 51 and 52 of the corresponding bearing sleeve, which are inmaximum spaced apart relationship with respect to each other, thismaking it possible to ensure a better support of each blade 36 andreducing the radial efforts exerted by the collars 51 and 52 on thecorresponding self-lubricating rings 34 and 35.

Finally, when each blade 36 is suitably disposed in the hub 26 so thatthe collars 51 and 52 of the corresponding bearing sleeve 48 are inabutment inside the rings 34 and 35 of the corresponding openings 32 and33, the metal connection elemetns 43 of the inner radial end of theloop-shaped root part 42 of the spar 39 is received between two metalrings 65 and 66 engaged around the driving sleeve 17, and by a singlepin 67, of which the shank passes through aligned bores in the flange 21of the sleeve 17, the central part 29 of the hub body 26 and the rings65 and 66, as well as the central passage of the connection element 43,and on the threaded end of which is screwed a cottered nut 68,connection of this connection element 43 to the hub body 26 and to theflange 21 of the driving sleeve 17 is ensured, the metal rings 65 and 66on either side of the connection element 43 ensuring distribution of thecentrifugal forces from one connection element 43 to the other.

Finally, a convex cover 69 fixed, for example by screws, by its outerradial edge on the periphery of the outer radial wall 30 of the hub body26, covers the assembly for collectively controlling the pitch of theblades 36 and protects the hub against the penetration of dust, stones,etc. . . .

In this embodiment, it is observed that the central part 29 of the hubbody 26 works little and serves only to transmit the driving torque fromthe driving sleeve 17 to the blades 36, in order to rotate the latter.However, the hub body 26 takes up part of the bending efforts. This isobtained thanks to the favourable disposition of the support collars 51and 52 of the bearing sleeves 48. Moreover, the rings 65 and 66 fordistribution of the centrifugal efforts avoid any considerable stress ofthe driving sleeve 17 and prevent the fixing pins 67 from bending.

This embodiment allows easy assembly and dismantling of the differentcomponents of the multi-blade rotor, which considerably facilitatesmaintenance operations and reduces the cost of maintenance. Inparticular, it is easy to proceed with changing a damaged blade 36without having to dismantle other elements, since it suffices todisconnect the corresponding boss 53 from the pin 63 by unscrewing thenut 64 and to disconnect the corresponding connection element 43 fromthe pin 67 by unscrewing nut 68. To this end, the cheek 57 may beperforated to allow access to the nut 68 without having to dismantle thecheek 57 and the plate 55 from the shaft 59, after disconnection of thebosses 53 from all the blades 36.

Referring to FIGS. 5 to 7, a process will now be described formanufacturing a blade such as 36 whose structure has been described withreference to FIGS. 2 to 4 and which is intended not only to equip amulti-blade propeller such as the one described previously and used astail rotor of a helicopter, but also to equip ventilators or fans,particularly for blowers and air-conditioning or gaseous heat exchangerdevices.

This process is carried out with the aid of two complementary halfmouldsof which the lower one 70 presents an inner impression whose formcorresponds transversely and longitudinally to the form of the lowerface of the blade 36, and of which the upper one 71 presents an innerimpression whose form corresponds under the same conditions to that ofthe upper surface of the blade.

Two elongated layers of fabrics of glass or KEVLAR fibers are firstlycut out, then two elongated layers of carbon fiber fabrics, each layercomprising two adjacent foils 37 I and 37 E on either side of alongitudinal median axis corresponding to the leading edge of the shell37 and adapted to form the lower surface part and the upper surface partof the shell 37 respectively. These layers of fabrics arepre-impregnated with a polymerizable synthetic resin, for exampleheat-setting. The foils 37 I formed by the two layers of glass or KEVLARfiber fabrics are then deposited in the impression in the lowerhalf-mould 70, leaving foils 37 E of these two layers outside the lowerhalf-mould 70, on the leading edge side, then the two layers of carbonfiber fabrics are deposited in the same manner on these two layers, sothat all the foils 37 I of the four layers of fiber fabrics forming theshell 37 are stacked in the lower half-mould 70, as shown in FIG. 5. Thepreformed filling mass 38 of cellular material or foam, of which thecut-out 41 opens upwardly, is then placed on this stack of foils 37 I.There is no difficulty in making this mass 38 in the desired shape in asecond mould arranged to this end and this operation does not requirefurther explanations. The reinforcing yoke 72 constituted by fabrics ofcarbon fibers preimpregnated with synthetic resin and preformed as a Vin separate equipment is positioned on the rear edge 38a of the fillingmass 38. Then the spar 39 constituted by an elongated leaf whose endpart is formed as a loop, is deposited in the cut-out 41 in thepreformed mass 38 in position in the lower half-mould 70. Manufacture ofelongated leaves of this type, intended in particular to constitute thewebs of the rotor blades of rotorcraft, is well known and does not needto be described in detail; it suffices to specify that leaves of thistype may be constituted not only with rovings of KEVLAR fibers, but alsowith rovings of glass fibers or carbon fibers. All these rovings may beagglomerated for example by impregnating the bundle which they form witha polymerizable resin, particularly, a heat-setting one. This knownprocess makes it possible to produce in particular thin, relativelynarrow, elongated leaves which present mechanical properties which areparticularly advantageous for making the webs of rotor blades ofrotorcraft: in fact, they combine a high mechanical resistance in thelongitudinal direction, enabling them to absorb the centrifugal forcesapplied to the blades of which they constitute the spars, withoutexcessive stress nor fatigue, with relative suppleness, in particular intwist about their longitudinal axis, which makes it possible to controlthe respective angles of attack of the blades by exerting on theelongated leaves torsional moments centred on their longitudinal axis,and of relatively low value.

In this precise example, the spar 39 is constituted by a singleelongated bundle of rovings which is folded on itself at the centre ofits length and of which the two end parts extending over about twothirds of each half of the bundle are coupled in order to constitute theportion of the spar intended to extend in the general part of the blade38, whilst the median part of the bundle, extending over about one thirdof each half on either side of the centre of the length of the bundle isintended to constitute the twistable, flexible, loop-shaped root portion42, after folding the bundle on itself in two equal halves. The bundleof rovings impregnated with heat-setting resin is therefore spread outin the cut-out 41 in the filling mass 38, then the foils 37 E of thelayers of fabrics of coating fibers are folded onto the mass 38 and thebundle of rovings, and the titanium or stainless steel sheet cover 40 isthen installed on the leading edge, as shown in FIG. 6. As shown in FIG.7, the upper half-mould 71 is then placed on the lower half-mould 70 andthe whole of their contents is polymerized by heat treatment, thisensuring integration of the leading edge cover 40 in the fabrics of therigidified shell and which adheres by the whole of its inner sufaceeither on the mass 38, or at the level of foils 37 E of the uppersurface part directly on the spar 39, of which the rovings areagglomerated.

The transition zone 47 and the hollow cylinder 46 of the blade root 44may be made simultaneously thanks to longitudinal recesses ofcomplementary shape made in that part of the half-moulds correspondingto the blade root and in which is disposed a hollow mandrel, traversedby that portion of the bundle of rovings constituting the loop-shapedroot 42. This mandrel, surrounded by the layers of reinforcing fiberfabrics 45 is disposed in the recess of the lower half-mould 70 abovethe parts of the layers of coating fabrics extending the foils 37 Ipreviously disposed in the impression of this half-mould 70. When thefoils 37 E are folded down, parts of coating layers extending the latterare folded down above the mandrel and the reinforcing layers 45. Thegeneral part of the blade 36, the corresponding portion of the spar 39,the blade root 44 and the twistable, flexible, loop-shaped root portion42 of the spar are thus moulded and polymerized in a single mould and bya single heat treatment.

In order to facilitate the loop arrangement of the central part of thebundle of rovings folded on itself, a wedge-shaped element may beprovided in the lower half-mould 70, around which the bundle of rovingsis spread. In its central part, the latter presents a thickness whichprogressively increases towards the centre of its length, as shown inFIG. 2. Similary, it may present a width which decreases slightly andprogressively from each of its two ends towards its central part, withthe result that the portion of the spar 39 housed in the general part ofthe blade 36 widens towards its outer radial end and thus opposes tearby the centrifugal force of the filling mass 38 and the shell 37.

We claim:
 1. In a variable-pitch multi-blade propeller intended inparticular to be used as tail rotor of a rotorcraft, and comprising:acentral shaft driven in rotation about a central axis, a hub rotatingwith said central shaft about said axis, blades, preferably in an oddnumber, each comprisinga shell with aerodynamic profile constituted byat least one layer of fiber fabrics with high mechanical resistancerigidified by a polymerized synthetic resin for impregnation, and havingan inner end which extends by a blade root, a filling body made ofcellular or foam synthetic material disposed in said shell, a sparhaving a longitudinal axis substantially parallel to that of said bladeand constituted by a single elongated leaf of fibers with highmechanical resistance agglomerated by a polymerized synthetic resin, ofwhich the major part is fixed in said shell and of which one inner endpart, emerging from said shell on passing through said blade root, formsa twistable and flexible root part by which said spar is connected tosaid hub, said blade root being fast with a pitch control member whichis adapted to exert on said shell a torsional moment, substantiallycentred on said spar longitudinal axis, when said pitch control memberis actuated by an assembly for collectively controlling the pitch ofsaid blades, and which is mounted to rotate in said hub about said bladelongitudinal axis and wherein (a) each blade is individually connectedto said hub by said spar root part which is arranged in the form of aloop having an inner end surrounding a single connection element boltedon said hub, (b) said hub has, for each said blade, two axiallyextending parts which are radially spaced apart from each other, and ineach of which there is an opening which is coaxial with the opening ofthe other hub part, and (c) said blade pitch control member is rotatablymounted in said two coaxial openings.
 2. The propeller of claim 1,wherein said hub has an annular flat inner radial part surrounding acentral opening made in said hub to allow the coaxial arrangment of saidhub about said central shaft, and each connection element is a tubularelement connected to said annular flat inner radial hub part by means ofa single pin having a shank with threaded end and axially traversingsaid tubular connection element.
 3. The propeller of claim 1, whereintwo rigid radial flat rings are disposed on either side of said bladeconnection elements, and about said central shaft, each said connectionelement has a central passage, and each said flat ring presents, foreach said connection element, a bore adapted to be aligned with saidcentral passage, and said connection element is connected to said hub bymeans of a single pin having a shank received in said central passageand in the corresponding bore of each said flat ring.
 4. The propellerof claim 1, wherein said hub is in the form of a dish comprising twocoaxial cylindrical walls radially spaced apart from each other andforming said two axially extending hub parts in which are pierced saidtwo coaxial openings which are of circular sahpe and each centred onsaid blade longitudinal axis and housing a coaxial self-lubricating ringfor rotatably mounting said blade pitch control member.
 5. The propellerof claim 4, wherein the inner diameters of the opening made in the innerradial wall of said hub and of the self-lubricating ring housed in thisopening are respectively less than the inner diameters of the openingpierced in the outer radial wall of said hub and of the self-lubricatingring housed in this opening, and said blade pitch control memberpresents two coaxial circular bearing surfaces centred on said blade,longitudinal axis, radially spaced apart from one another so that by itsouter radial bearing surface, of which the outer diameter corresponds tothe inner diameter of said outer radial ring, and by its inner radialbearing surface, of which the outer diameter corresponds to the innerdiameter of said inner radial ring, said pitch control member abutsrespectively inside said outer and inner radial rings.
 6. The propellerof claim 5, wherein said pitch control member is a bearing in the formof a sleeve, traversed by said spar root part, covering said blade rootand connected to the latter, for example by gluing, and of which each ofthe ends is surrounded by one of said two bearing surfaces.
 7. Thepropeller of claim 6, wherein said blade root comprises a hollowcylinder whose inner end is truncated in form and a transition zoneconnecting said hollow cylinder to said blade shell, an said sleevecomprises an outer cylindrical part of large diameter surrounding saidblade root hollow cylinder and connected by an intermediate truncatedpart, by which said sleeve bears against said hollow cylinder truncatedinner end, to an inner cylindrical part of small diameter.
 8. Thepropeller of claim 7, wherein said two bearing surfaces are eachconstituted by one of two collars of spheroidal shape each formed on theouter surface of a corresponding sleeve end, and in one piece with saidsleeve which is made of metal.
 9. The propeller of claim 5, wherein saidblade pitch control member also comprises, between said two bearingsurfaces, a lateral boss connected by a ball joint to one of severalaxial lugs, in a number equal to the number of blades of the propeller,of a pitch control plate displaced axially, parallel to said axis ofrotation, by a shaft for collectively controlling the blade pitch andmounted to slide axially in said central shaft which is tubular.
 10. Thepropeller of claim 9, wherein said control plate is an annular, convexplate connected to said shaft for collectively controlling the bladepitch by a cheek of truncated shape mounted by conical fit on saidshaft.
 11. The propeller of claim 9, wherein each of said axial platelugs presents a ringed bore in which is received a ball joint retainedon a shank fixed to the corresponding lateral boss.
 12. The propeller ofclaim 1, wherein it comprises, in addition, a driving sleeve, adapted totransmit the driving torque from said central shaft to said hub andsurrounding said central shaft, being coaxial to the latter an rotatingwith the latter and comprising an annular, outer radial flange forconnection to an annular radial part of said hub.
 13. The propeller ofclaim 12, wherein the connection of said driving sleeve flange to saidhub annular part is ensured by pins regularly distribruted about saidaxis of rotation, in a number equal to the number of blades of thepropeller, and of which each simultaneously ensures retention, on saidhub annular part, of a connection element of said spar root part of ablade, in cooperation with a nut screwed on a threaded end of a shank ofthe corresponding pin which passes through bores formed in said flangeand said hub annular part and which are aligned with a central passageof said connection element.
 14. The propeller of claim 12, wherein saiddriving sleeve is maintained in axial position about said central shaftwith the aid of a tapped ring screwed at the end of said central shaft.15. The propeller of claim 12, wherein said central shaft bears outergrooves by which it is in mesh on the one hand with inner grooves on abevel gear and on the other hand with inner grooves on said drivingsleeve, separated from said bevel gear by a seal-holder, and an axialprestress device maintains a predetermined axial bearing force of saiddriving sleeve against said seal-holder and said bevel gear.
 16. Thepropeller of claim 15, wherein said driving sleeve is maintained inaxial position about said central shaft by means of a tapped ringscrewed on the end of said central shaft and wherein said prestressdevice comprises screws, screwed in tappings passing through said tappedring parallel to its axis, and each of which has a shank with an endshaped as a bearing stud applied against a bearing surface at the bottomof a notch in the axial end face of said driving sleeve which isopposite said seal-holder.